Divided chamber engines which use an auxiliary or precombustion chamber to achieve combustion control or charge stratification have been produced in both diesel and spark ignition forms. Automotive type diesel engines commonly utilize unscavenged precombustion chambers while spark ignition engines have been built with both unscavenged auxiliary ignition or precombustion chambers and with ignition chambers that are scavenged by an auxiliary intake valve. In either case, fuel may be introduced into the auxiliary chamber with an injector or through the auxiliary intake valve if provided.
Combustion in the prechamber or auxiliary chamber of a divided chamber engine generally occurs at an airfuel ratio that is richer in fuel than the overall airfuel ratio for the engine. With spark ignition engines, this can mean that high levels of carbon monoxide or unburned hydrocarbons may be present in the prechamber combustion products. With diesel engines, higher than average levels of particulates may be present in the prechamber combustion products. During the exhaust blow-down process, cylinder pressure drops rapidly, allowing the auxiliary chamber combustion products to pass into the main combustion chamber and through the open exhaust valve to the engine exhaust system, adding to the level of emissions in the engine exhaust.